Users need to be able to have multiple active login sessions because they often have to use CircleCI on both desktop and mobile devices.
I often find myself having to login on my phone in order to re-trigger failed builds (sometime from flaky tests, and more frequently from random CircleCI timeouts). This apparently logs me out of my desktop session. Given that the unauthorized page doesn't provide users with a login link or redirect back to the desired page after logging in (https://circleci.canny.io/cloud-feature-requests/p/redirect-to-login-page-when-viewing-pages-while-unauthenticated), managing CI from multiple devices is a very frustrating experience.
Neither of these should be issues in an age where people have to manage issues on their computer at work and during off hours on their phone. The initial response I received from support claimed that the single session is done intentionally for security purposes, but plenty of other secure applications allow for multiple sessions without incident.